It is well known to connect two or more telephone sets, for example, in different rooms of a residence, to the same telephone subscriber line, thus enabling calls to be originated or terminated at each telephone set or permit two or more people in the residence to participate simultaneously in a telephone conversation via the telephone subscriber line. A disadvantage of such an arrangement is that the privacy of a person using a first telephone set may be lost due to another person taking a second telephone set, connected to the same telephone subscriber line, off-hook (lifting the handset). While in some circumstances a click may be audible to the person using the first telephone set when the second telephone is taken off-hook, this does not provide a convenient or reliable indication that privacy has been interrupted. Accordingly, it is desirable to provide a more reliable indication at one telephone set of the hook state of another telephone set connected to the same line.
Such an indication is also useful even when the first telephone set is on-hook, as it can then serve to indicate to a prospective user of this telephone set whether or not the telephone subscriber line is already in use by another person using one of the other telephone sets connected to this line.
Furthermore, it is desirable for a user of the first telephone set to be able to place this in a hold state during a telephone call, to go to a second telephone set connected to the same line to continue the call, and to have the first telephone set automatically release itself from the hold state when the second telephone set is taken off-hook. This is referred to as remote release from hold.
Prior art devices take advantage of the fact that a change in the voltage between the two wires of the telephone subscriber line can be detected and used to take an appropriate action, for example to provide an indication at a first telephone set whether another telephone on the same telephone subscriber line has gone off-hook or to effect a remote release of hold.
As an example of such a prior art device, the reader is directed to U.S. Pat. No. 5,287,404, issued Feb. 15, 1994, naming Pepper et al as inventors, the disclosure of which is hereby incorporated by reference.
FIG. 1 illustrates a prior art arrangement in which three telephones are shown connected to the same telephone subscriber line within a residence wherein said telephone subscriber line is directly connected to a central office using the conventional analog telephone subscriber line interface. In such a prior art arrangement, the central office 10 includes a subscriber line interface circuit (SLIC) 20 to which is attached telephones 52, 54 and 56, within the residence 50, via a telephone subscriber line 30. For example, let us assume that telephone set 52 contains a device for detecting changes in the voltage and current (VI) characteristic of the telephone subscriber line according to the prior art. If telephone set 56 goes off-hook, then set 52 can indicate to a user, for example, via a light, that set 56 has gone off-hook.
By way of example, let us assume that central office SLIC 20, using an open circuit voltage of 52 V, generates a potential of 52 volts between the two wires of the telephone subscriber line 30 when all telephone sets are on-hook. When telephone set 56 goes off-hook, telephone set 56 starts to draw current from the central office SLIC, via the telephone subscriber line, decreasing the voltage potential between the two wires to approximately 15 volts or less at the residence. The telephone set 52 senses this drop in potential which indicates that another extension connected to the telephone subscriber line has gone off-hook. Furthermore, if telephone 54 subsequently goes off-hook in parallel with telephone set 56, the current drawn from the central office SLIC by telephone 56 decreases, and if telephone set 56 also contains a device for detecting changes in the voltage and current characteristic of the telephone subscriber line, telephone 56 senses this drop in current or potential and indicates that another extension connected to the telephone subscriber line has gone off-hook.
While such features were designed for conventional (e.g. POTS) telephony service provided by a central office switch, it should be appreciated that other telephony services exist. For example, wireless (e.g. cellular) telephony has been proposed as a means of providing telephony services to a residence. In order to save costs associated with mobile cellular telephony, fixed wireless access (FWA) systems have been proposed. U.S. Pat. No. 4,658,096 naming West et al as inventor, describes an interface system for connecting a conventional telephone to a cellular transceiver. Such an interface system includes a SLIC for providing central office subscriber line interface features to a conventional telephone connected to the interface. However, there exists a need for providing more advanced features to such a system.
It is advantageous to add a handset (hereafter referred to as the primary handset), keypad, speaker, and display directly to the subscriber unit containing the SLIC. This transforms the subscriber unit into a wireless terminal which can also provide central office subscriber line interface features to additional conventional telephones via the built in SLIC. However, when this primary handset goes off-hook, there is no affect on the telephone subscriber line attached to the SLIC interface within the subscriber unit. Thus, a telephone set connected to the subscriber unit will not be able to detect the primary handset going off-hook. Therefore there exists a need for regulating the VI characteristics of the telephone subscriber line attached to the SLIC in the subscriber unit.